Plug Building

Discussion in 'Boat Design' started by Trick Powerboats, Nov 18, 2012.

  1. tunnels

    tunnels Previous Member

    concave hull was less efficient than a convex !!!please exspand you answer !! efficent in what way ??
    Lets speak about strength ,is a concave shape bottom more strong than a convex ?? :confused::eek::mad::confused::eek::mad:

    outies are stronger than innies how do you come to this conclussion ??? why please exlpain in simple easy to understand why why why why ????:confused::eek::confused::eek:
     
  2. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    My gut feeling is that a concave ( outie ) is stronger, because the water pressure puts the hull structure mostly in compression, whereas convex ( innie ) relies on the tensile strength of the glass to prevent the hull from 'collapsing'.

    This is only my theory, and I would love to get more information from qualified contributors.
     
  3. tunnels

    tunnels Previous Member

    Ok let get the shape sorted first !! concave is standing at the front of the boat and concave is the panel is curved inwards
    Convex is the bottom panel bows outwards
    In the case of bottom panels and also top side panels a concave would be stronger than a convex ,The pressure is always wanting to push inwards never pushing outwards !!!Right ???? :confused::?::eek:
     
  4. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Whoops, got those directions mixed up in the last post -
    ConCAVE is inwards ( like a Cave )
    ConVex is outwards.

    So a Convex hull shape is the outie - bows outwards.

    So, according to my personal theory - the water pressure from the outside is trying to push the hull inwards ( especially under power ).

    The outwardly ( convex ) hull shape tries to become compressed, but a concave tries to stretch. An eggshell is much harder to break as it is a convex shape. If an eggshell was Concave ( innie) then the strength would rely on the shell not stretching inwards.

    In the picture below, the dam is resisting the huge amount of water by presenting a Convex (outward rounded shape) to the water.

    Any qualified opinions would be appreciated.
     

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  5. Trick Powerboats
    Joined: Apr 2008
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    Location: Al Raha Beach

    Trick Powerboats Junior Member

    Ok Ok Ok.....enough with the bickering.....

    Just to be clear, there are concave AND convex shapes on the bottom and sides. The sides starting at the transom, go from convex to concave at midship. The bottom at the transom and working out from the keel goes from convex to concave at the chine and diminishes and you move forward.

    The schedule as follows
    Sides....
    Gel + 300gm csm + 600gm 45* + 600gm 0/90 + core + 300gm csm + 600gm 0/90 + 600gm 45*

    Bottom....
    Gel +300gm csm + 600gm 45* + 600gm 0/90* + 600gm 45* + core + 300gm csm + 600gm 0/90 + 600gm 45* + 600gm 0/90

    I'm not sure what 600 gm biaxial works out to in ounces for those of you working in lbs and not kgs. Sorry

    Jeff
     
  6. rwatson
    Joined: Aug 2007
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    rwatson Senior Member

    Sorry - I hope I wasn't contributing to any 'bickering', I was enjoying the learning experience.

    Just out of curiosity though, what sort of engineering analysis did you do on the hull layup ?

    Did you get into finite-element level modeling, or just adopt some general scantling standards ?

    Cheers
     
  7. Trick Powerboats
    Joined: Apr 2008
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    Trick Powerboats Junior Member

    Its a slight variation on a lamination schedule that I've been using for years. I've build alot of boats this way and only had one delaminate. And that was a scrim failure on one piece of honeycomb.

    Jeff
     
  8. tunnels

    tunnels Previous Member

    Ok in more than 35 years of building all kinds of boats, plane parts, acid tanks and swimming pools ,yachts to 147 foot powerboats , formular 0ne tunnels ,i never had even the slightest hint of a faulure no delaminate, no breakages and light and strong and would never use carbon for anything !!.
    OOOPS forgot surf life saving boats that go out in any kind of weather and had crews return with broken legs and the boats never missed a beat week in and week out !!
    If you are comfortable with the laminate you spec-ed OK !! I was only suggesting what i would have used .
    Just remember that hull shape increases the strength of the panels and with a concave shape you have it made . even with no cores at all just a good frame inside it would be much more ridged than a standard shaped hull with a convexed bottom and top sides . :D:D:D
     
  9. tunnels

    tunnels Previous Member

    Most every designer i know of and all the ones i never meet yet do not take into account shape as a way of making panels strong ! its always cores and glass that are there prime way of increasing stiffness never shape .
    Car designers have used shape for many years . every panel on any car has shape in lots differant directions and creases and channels and grooves and bumps and bulges etc etc .
    Boat designers havent shed there blinkers yet and still living in prehistoric times . Some cant smell the flowers because they never even found the flower bed yet !!.
    I made a completely new solid glass transom for my own boat and the glass is just 8 mm thick , i used shape to make it strong and ridged to hold the motor and all the extra forces from the new motor !! the horse power has increased to 115 hp and a 25 inch shaft ,not the older 20 inch and a tired 70 hp motor that it used to have !!.
    When panels have natural stiffness from there shape less materials are needed !! but what is used needs to be better than just the usuall woven roving and chopped strand matts of yester Year!!
    I would love to see the results of what a car designer could andwould do to make a new boat :eek::confused::) ,
     
  10. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Hey I had no intention of starting a pissing contest here. The concave/convex thing could be a semantic problem. ...........Take a soccer ball or basketball and cut it in half. If you look at the hemispheric object from the outside it has convex contours. If viewed from the inside, where the inflation air used to be, you are looking at a concave contour.

    From a purely analytic standpoint we consider the extreme fiber. That will be the surface farthest away from the applied load. Think of a piece of lumber suspended across a pair of saw horses. Place a weight on the top of the lumber plank. The bottom surface of the plank will have its fibers tending to pull apart. That is tension loading of the fibers. The top of the plank will have the fibers in compression, that is the fibers are being jammed together. Somewhere about the middle of the plank thickness there will be neither tension nor compression which implies no load at all. A load on a surface that is concaved exaggerates the tension on the fibers farthest from the applied load whereas the convex surface, loaded from the outside, does not exaggerate the extreme fiber tension as much. Most materials, including resins, can endure more loading in compression than in tension.

    Sheeesh! I apologize for even mentioning that bit of techno-babble in the first place. Of course the builder can use convex shapes. Judging from the appearance of the plug and its obvious good craftsmanship, I reckon that the builder is going to be plenty good at arriving at sufficient, experimentally determined, scantlings. Laying up some test strips, with representative curves, applying loads to the inside then the outside will be useful enough to settle whatever questions there may be.
     
  11. Trick Powerboats
    Joined: Apr 2008
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    Trick Powerboats Junior Member

    We've finished the planking and we're gonna do a little fairing before adding the strakes and sprayrail. We'll hit with some black spray can primer and long board it. After the stakes and spray rail are added We'll hot coat it with some GP resin to lock in the MDF fibers, use q cell to fill any obvious lows and start the LB again...

    Jeff
     

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  12. tunnels

    tunnels Previous Member

    wow looks good !

    That picture will raise a few eyebrows !!:eek:
    Wow flaired hulls have a look all of there own , i havent seen one like that for years and years ,used to be all the rage back in the 60s flaired bows and big fore decks . Good luck with the long boarding . lots sore muscles you didnt know you had !!.
    Used to pressure pot spray highbuild indercoat loaded with q cells !!,went off quickly and was really good to sand ,each coat a differant color so knew exactly what was going on and getting sanded off .
    How many strakes each side ?
    stepped back from the front ??
    and all run to the transom ?? :p
     
  13. Trick Powerboats
    Joined: Apr 2008
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    Location: Al Raha Beach

    Trick Powerboats Junior Member

    Here are some pics of the base for the deck/shear jig. We started laying MDF on it today. Once that is complete we can start shaping it as we go. We decided to go this route instead of cutting everything to a pre-determined plan on the CNC. We wanted alot of flexibility once we had a "feel" for the shape and scale of things.
     

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  14. tunnels

    tunnels Previous Member

    As long as you have strong thoughts and ideas of what you want and can make it all work go for it , Cnc is a way to get some basic things like decks and sides quickly cut and in place to work away from if you have the access to one . the more you use the cnc the simpler life can be . On projects i worked on i take every advantage possible to get work dont on the cnc even small time consuming things use it . Its just another tool but its only as good as the driver !! you have a driver with fore sight and good skills you can make it happen quickly . :D
     

  15. Trick Powerboats
    Joined: Apr 2008
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    Location: Al Raha Beach

    Trick Powerboats Junior Member

    Ok,

    All the plug work is finished.

    Built as follows

    3 layers of 6mm MDF planking, wood glued between layers, over plywood jig frames.

    We coated the faired MDF plug with gp resin to "lock" the MDF fibers and provide additional waterproofing. We shot the faired plug with duratec surface primer, sanded and wet sanded through 1000 grit. We polished the plug and continuously waxed (mirror glaze #8) over the course of three days til 20 coats were finished.

    The console......
    We rolled on 2 coats Norpol green tooling gel and backed it up with black. Did the lamination with Reichold vinylester resin.

    The plug stuck, had to be cut out and we're still cutting. Any ideas?

    I hate to shoot the hull with PVA but I will to prevent this nightmare from happening again.

    Suggestions appreciated

    Jeff
     
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